Sentence Correction (SC) | Re: No less impressive than the invention of the light bulb by Thomas Edis

I still do not understand how were is correct and was isn't.
Why "invention" is not a subject here??
if it was inventions then we might have used were.

for eg. The day of Raj and Ricky's graduation is special. --> correct
The day of Raj and Ricky's graduation are special. --> dont think correct
The days of Raj and Ricky's graduation are special. --> correct

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